Maser



Feb. 27, 1962 0;]. BOLEF ETAL 3,023,367 I MASER \l Filed May 29, 1957 2 Sheets-Sheet 1 Fig.|.

Energy Magnetic Field Feb. 27, 1962 1. BOLEF ETAL MASER 2 Sheets-Sheet 2 Filed May 29, 1957 ATT RNEY fl i States Patent MASER Dan I. Bolef and Peter F. Chester, Penn Township, Allegheny County, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed May 29, 1957, Ser. No. 662,415 3 Claims. (Cl. 330-4) This invention relates to devices for amplifying or generating electromagnetic energy'and, more particularly, to amplifying devices employing solids, liquids or gases for generating microwave energy by stimulated emission of radiation.

Generally speaking, masers" may be defined as devices for amplifying or generating electromagnetic energy by utilizing molecules in the excited state'iof a microwave transition. Interaction between these excited molecules and a microwave field produces additional radiation and hence amplification by stimulated emission..

The operation of masers such as the one described herein is dependent upon the fact that in certain paramagnetic materials a given electron may be in one of three or more different energy states. These energy states may be thought of as arising from the interaction of the electrons with internal or external fields. They may, therefore, be referred to as electron spin states. .An electron may undergo transitions between states as a result of interaction with external microwave energy of an appropriate frequency. In this respect, electrons in a low state may be transferred to a higher state when paramagnetic material under the influence of a magnetic field is subjected to a microwave field having a frequency 7 corresponding to the energy difference, between the twostates. If the system of electrons is in a state of thermal equilibrium, corresponding to an excess electron population in the lower over the upper spin states, the paramagnetic material will absorb energy. When the situation is reversed, however, and there is an excess electron population in an upper state over a lower state, elec-' trons in an upper state will release energy to incident microwave energy of the correct frequency, thereby producing amplification. If the frequency of the incident microwave energy is not of the correct frequency to be amplified, or if no signal is applied to the paramagnetic material, electrons in upper states by interaction with their surroundings, will'revert to a lower energy level over a time interval called the spin-lattice relaxation time. V a

It is an object of this invention to provide anew and improved maser for generating or amplifying electromagnetic energy.

Another object of the invention is to provide a solid state frequency converter.

A further object of the inventionis to provide means for amplifying or generating electromagnetic wave energy by employing paramagnetic materials having at least three electron spin energy levels. e

A still. further object of the invention is to provide t means for increasing the frequency at which a paramagne'tic' material will release electromagnetic energy.

Another object of the invention is to provide a solid state microwave frequency converter which operates superregeneratively. t

The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings which form a part of this specification and in which:-'

FIGURE 1 is a graphical illustration of three of the levels existing in a paramagnetic material showing how -the levels change as a function of applied magnetic field;

"ice

FIG. 2 is an illustration of one embodiment of the invention; j

FIG. 3 is a cross-sectional view taken along line III-- III of FIG. 2;

FIG. 4 is a cross-sectional view taken along the line IVIV of FIG. 2 showing the first microwave cavity of the invention;

' FIG. 5 is a cross-sectional view taken along line V-V netic material. Not all paramagnetic materials have three separate and distinct energy levels; however, examples of materials which do have three or more energy levels and are otherwise suitable for purposes of 1 the present invention are: v

(a) Gd(C2H5SO4)3-9D2O (Or to One part in 200 with La(C H SO .9D- O (or 91-1 0).

(b) NiSiF .6H O diluted 1:20 or more with ZnSiF .6H O (c) Diluted chromium can salts e.g.

(NH )CR(SO .l2H O or CrK(SO .12H O.

(d) Cr in potassium cobalti-cyanide or in A1 0, crystals or other host crystals.

Some of the electrons in the paramagnetic material will be in the lower energy state E shown in FIG.--1 while the remainder will be in the higher energy states E and E The paramagnetic material is in a normal or relaxed condition when the population distribution is such that there are more electrons in state E than in state E and more electrons in state E than in state E Electrons in the various energy states can interact with a microwave radiation field of appropriate frequency and either'absorb energy from the radiation field while advancing to a state of greater energy or, under the influence of the radiation field, can give up some of their energy and drop to a state of lower energy. The amount of energy thus transferred is related to the frequency of the radiation field by the equation:

upper'f' lower= where the symbol h is Plancks constant and v isthe frequency.

a fan electron in the lower energy state E is under the influence of a magnetic field H for example, and is subjected to a microwave field having a frequency corresponding to the energy difference between the states E and E it will absorb energy and advance to the upper energy state E Similarly, if an electron in the lower energy state E is under the influence of a magnetic field and is subjected to a microwave .field having a frequency corresponding to the energy difference between states E and E it will advance to the highest energy state E The energy state. All such systems, when allowed to come to thermal equilibrium, have more electrons in the lower energy states and, hence, are absorptive.

In order for the paramagnetic material to be emissive, there must'be an excess of electrons in an upper energy state over that in a lower state. Thus, if there is an excess-of ele'ctrons in energy state Eg over state E the electrons in state E can releaseenergy and fall to energy state E Likewise, ifthere' is an excess of electrons in energy state E over states E or- E the electrons instate E will release energy while falling to one of the lower energy result'in a condition wherein there are more electrons in the energy level-E than there are in level E Thus, a steady state emissive condition is established between states E and 3 E If: the paramagnetic material is now subjected to microwave energy having a frequency correspondmg to the, energy difference between states E and E theelectrons falling from state E to state E5 will release energy to the. incident microwave field, thereby producing amplification. Under other conditions, depending upon the relaxation timesbetween states E3-E2 and E E the population in state E can be made-to exceed that in state 13,, and stimulated emission will be possible at a frequency corresponding to. the energy difference between states E and E.

If a paramagnetic material having three or more spin states is' subjected to saturating microwave radiation under .the influence of a low steady magnetic field H then energy states E and E may become equally populated. Then, ifthe paramagnetic material is transported to a higher magnetic field H where stimulated emission is'induced, theincrease in fieldresults in a corresponding increase inspacingbetWeen energy levels. Thus, three new energy levels now exist. This condition results in a corresponding; increase in the-frequency of emission between states E andE or between E and E. If the field H ishighenough, the frequency of'emission may exceed the frequency corresponding to the energy difference between states E and E3 in, the low magnetic field H (i.e. the

frequency of the saturatingradiation). As shown in the drawing the frequency of emission Vi between states E and'E at H is greater than the frequency V at H It is, of course, necessarythat the paramagnetic material be transported from the point-where saturation occurs to the point where emission occurs in a time that is short compared with the spin-lattice relaxation-time of. the paramagnetic material.

One embodiment of the invention for effecting theproc ess described abovetis shown in FIG. 2 and comprises. a

disc 10 of paramagnetic material, or containing some paramagnetic atoms or molecules, which is'made to rotate about an axis 12. If preferred, only the periphery of disc 10 need be formed from paramagneticmaterial, or a hoop 11 of. paramagnetic material may be affixed'to the periph- I cry of disc 10 as'shown in FIGS. 3 to 5., A portion of the periphery of disc 10 rotates betweenthe opposite poles of a maguet14 for applying asteady magnetic field'across the-paramagnetic material. This has-the effect of producing a thermal distribution of population among the energy levels. After passing through the field produced by magnet 14, the periphery of disc 10 passes through a resonant cavity 16 which is under the influence of a magnetic field H produced by magnet'18. Thereafter, the paramagnetic material passes-between the opposite poles ofathirdmagnet m. At-thetendot magnet ZOisa-sec- 0nd resonant cavity 22 throughvwhich the periphery of. disc I 10 passes. Electromagnetic energy, of appropriate frequency, which will saturate the energy levels E and E of the paramagnetic material on disc 10 is fed to cavity 16 through waveguide 24 from a generator, not shown, to produce a standing wave condition within the cavity. Electromagnetic energy having a frequency different than the electromagnetic energy fed to cavity 16 may be fed into cavity 22 through waveguide 26 and will leave the cavity as amplified electromagnetic energy through the same waveguide or through waveguide 28. As shown by the polarity markings in FIGS. 3, 4 and 5, the magnetic fieldsproducedby magnets 14,18 and20 are all in the same direction. It is, of course, necessary that the cavities 16 and 22 and the magnetic .fields in which they are situated'be tuned to the frequencies of the respective sginals fed to waveguides 24 and 26.

In accordancewith the description given above, the electromagnetic energy fed'to-cavity 16 through waveguide 24 has a frequency corresponding to the energy difference between the'states E and'E infield H The'magnetic field H produced by magnet 18 may be smaller than the magnetic field Hgproduced by magnet 20. Furthermore, the magnitude of the field produced by magnet 20 determines; the frequency at which stimulated emission will occurand this frequency, in turn, must be the frequency of the electromagnetic energy fed to cavity 22 through waveguide:26. It is, of 'course, necessary that the speed of rotation of disc 10 be such-that the time taken to travel from cavity 16 tocavity 22 is less than the spin-lattice relaxation timeso that the paramagnetic material is in an emissive condition'when it reaches cavity 22.

Iti-is to. be understood that the rotating disc 10 is only one of several ways to transport the paramagnetic material through the various magnetic fields and resonant cavities. As an alternative, paramagnetic material may be placed on an endless tapewhich passes through elements correspondingto magnet 14, cavity 16, magnet 18, magnet 20 andcavity 22. A still further method would be toemploy an inert fluid, containing the paramagnetic substance-:in solution or suspension which is circulated through tubes by a pump so that the electrons undergo the same, processas inthe mechanical arrangement described above. Liquid refrigerants such as liquid helium, hydrogen, argon, or nitrogen could be used as the fluids, thus combining spin transport and cooling functions. For a detailed description of the alternate devices described above, reference may be-had to our copending application,.Serial;No. 656,093, filed April 30, 1957, and assigned tothe assignee of the present application.

Anembodiment of'the invention which operates superregeneratively is shown in FIG. 6. This embodiment of the invention isverysimilar to that shown in FIGS; 1 to 5 and comprises a disc 210 which rotates about an axis 212. A point on the periphery of the disc, as was the case-with the embodiment shown in FIGS. 2 to 5, first passes through the magnetic field produced by magnet 214 and then passes to the-resonant cavity 216 which is under the influence of a magnetic field produced by magnet- 218.. Electromagnetic energyof a frequency corresponding to energy difference betweenthe states E and E is fed into-cavity-216* through a waveguide 224 to produce a standing wave condition. After passing through cavity 216, the paramagnetic material passes-to a magnet 220 andcavity 222 where incident electromagnetic energy-enteringwaveguide- 226 leaves as amplified electromagnetic energy through the same waveguide or through waveguide 228. As was the casewith the embodiment of the invention showninFIGS. 1 to 5, the magnetic field H producedby magnet 218 may be smaller than the magnetic field HR produced by magnet 220.

The difference between the embodiments of the invention' shown in FIGS;'2 and 6 isthatziu FIG: 6 the paramagnetic material'is: placed around theperipheryof the rotating-disc 210:in; diseretemndscparate sections 23.0.

Sections 230' must be widely enough spaced so that only one can be in the cavity 222 at a given time. If the jecting the paramagnetic material to. a second magnetic density of excited spins in any section is greater than that necessary to cause oscillation buildup in the cavity due to noise or incoming signal, the device can operate superregeneratively.

The separate sections 230 are moved through cavity 216 over a time interval which is long compared to the spin-spin relaxation time; while in cavity 216, they are subjected to the saturating microwave field which equalizes the electron sp'm population in states E and E They are then transferred to cavity 222 where noise or incoming signal through waveguide 226 will cause oscillation buildup in the system. The superregenerative action involving quenching or a cutoff comes automatically because of the physical removal of the separate sections 230 from the cavity 222. 1

It is to be understood that in each of the embodiments of the invention shown and described above, it will usually be necessary, depending upon the paramagnetic material used, to, operate the maser at a very low temperature. Accordingly, some means must usually be provided for subjecting the paramagnetic material to this low temperature during operation of the maser. The cooling system used in each case will, of course, depend on particular requirements.

It will be apparent that the device described herein could use a paramagnetic material having at least two electron spin levels in a magnetic field, and in which hyperfine interaction gives rise to further splitting of the electron energy levels. In this latter case, the frequency of the energy fed to the two cavities would have to' be adjusted to that corresponding to the difference in energy between the levels in question.

Although the invention has been shown in connection with certain specific embodiments, it will be apparent to those skilled in the art that various changes in form and arrangement of parts may be made to suit requirements without departing from the spirit and scopeof the invention.

We claim as our invention:

1. In apparatus for amplifying electromagnetic wave energy by stimulated emission of radiation, in combination, a continuous source of moving paramagnetic material moving in a predetermined path at a predetermined speed, said paramagnetic material. having first, second and third discrete energy levels of electrons, said second level being higher than said first level, said third level being higher than said second level, the separation between said energy levels being dependent upon the magnitude of an external magnetic field applied to the paramagnetic material, first and second spaced resonant cavities located in predetermined position with respect to said path and through which the source of paramagnetic material passes as it moves along said path, means adjacent the first resonant cavity for subjecting the paramagnetic material to a first magnetic field of predetermined direction and of a predetermined first magnitude while; the paramagnetic material is in the first resonant cavity, means for continuously feeding electromagnetic wave energy of a first predetermined frequency and of at least a predeterminal amplitude to said first resonant cavity, the spacing between the first energy level and the third energy level being such that the electrons in said paramagnetic material while in said first resonant cavity undergo transitions between said first energy level and said third energy level while subjected to electromagnetic wave energy of said first frequency and to the first magnetic field of said first magnitude, said predetermined speed being such that the movement of the paramagnetic material from the first resonant cavity to the second resonant cavity is short compared to the spin-lattice relaxation time of electrons between the third energy level and the second energy level, means adjacent the second resonant cavity for subfield-of a second predetermined magnitude greater in magnitude than the first magnetic field and having a direction substantially parallel to the direction of the first magnetic field as the paramagnetic material passes through the second resonant cavity, the spacing between the second and third energy levels in the second resonant cavity being greater than the spacing between the first and third energy levels in the first resonant cavity, and means for conducting electromagnetic wave energy of a second predetermined frequency to' be amplified to and from the second resonant cavity, the second frequency corresponding to the transition frequency between the third and second energy levels at said second magnetic field magnitude whereby electrons in the third energy level giveup energy to the electromagnetic wave of said second frequency as they revert to the second energy level thereby amplifying the wave energy of second frequency, the total length of said predetermined path and said predetermined speed being chosen with respect to each other and the paramagnetic material whereby paramagnetic material after leaving the second resonant cavity traverses a portion of at least a predetermined length of the path in returning to the first resonant cavity, the last-named paramagnetic material remaining in said portion at least a sulficient time to substantially produce a thermal distribution of the electron population between the energy levels.

2. In apparatus for amplifying electromagnetic wave energy by stimulated emission of radiation, in combination, a continuous source of moving paramagnetic material moving in a predetermined path of a predetermined speed, said paramagnetic material having first, second and third discrete energy levels of electrons, said second level being higher than said first level, said third level being higher than said second level, the separation between said energy levels being proportional to the strength of an external magnetic field applied to the paramagnetic mate- 'rial, first and second spaced resonant cavities through which the paramagnetic material passes as it moves along said path, means disposed adjacent the first resonant cavity for subjecting the paramagnetic material to a first magnetic 'field of a first predetermined magnitude and direction as it passes through the first resonant cavity, means for continuously feeding electromagnetic wave energy to said first resonant cavity of at least a predetermined amplitude and having a first predetermined frequency, the spacing between the first energy level and the third energy level being such that the electrons in said paramagnetic material while in said first resonant cavity under go transitions between said first energy level and said third energy level whereby an excess of electrons is created in the third energy level, relaxation of electrons from the third energy level to the second energy level producing an excess of electrons occupying the second energy level, the speed of movement of the source being such that the paramagnetic material is transferred from the first resonant cavity to the second resonant cavity in a time which is short compared to the spin-lattice relaxaion time between the second and first energy levels, means adjacent the second resonant cavity for subjecting the paramagnetic material to a second magnetic field of a second magnitude greater in magnitude than that of the first magnetic field as theparamagnetic material passes through the second resonant cavity, the direction of the second magnetic field being substantially parallel to that of the first magnetic field, the magntiude of the second magnetic field being sufiiciently large that the spacing between the second and first energy levels in the second resonant cavity is greater than the spacing between the first and third energy levels in the first resonant cavity, and means for conducting electromagnetic wave energy of a second frequency higher than said first frequency to and from said second resonant cavity, the electrons of said paramagnetic material whileinthe second resonant cavity: having transitions between. the second, and first energy levels at said second frequency and second'magnetic field magnitude, at least some of the-electrons in the second energy level while the paramagnetic materialis in the second resonant cavity reverting to the first energy level and giving up-energy to'the electromagneticwave energy of said second frequency thereby amplifying said last-named electromagnetic wave energy, the total length of said predetermined path and said predetermined speed being chosen with respect to each other and the paramagnetic. material whereby paramagnetic material after leaving the second resonant cavity traverses a portion of at least a predetermined length of the path in returningto the first resonant cavity, the last-named paramagnetic material remaining in said portion at least a suflicient time to substantially produce athermal distribution of theelectron population between the energy levels.

3. In apparatus for superregeneratively amplifying elect tromagnetic Wave energy by stimulated emission of radiation, in combination, a continuous source of amoving material moving in a predeterminedpath at a-predetermined speed, discrete similar sections of a paramag netic material carried atsubstantially equally spaced intervals by said moving materiahlsaid paramagnetic material having-three discrete levels of electrons including an upper level, a lower level, and an intermediate level, the separation between said energy levels being proportional to the strength of an external magnetic field applied to the paramagnetic material, first and second spaced resonant cavities through which the paramagnetic sections pass as the moving material moves along said path, first magnetic field producing means disposed in predetermined position with respect to the first resonant cavity for subjecting the sections of paramagnetic material carried by the moving material to a transverse magnetic field of predetermined direction and of a first magnitude as the sections move into position within the first resonant cavity, means for feeding electromagnetic wave energy of a predetermined first frequency and of at least a predetermined. amplitude into said first resonant cavity at least during the time intervals during which any substantial portion of a discrete section of paramagnetic material is within the first resonant cavity, the spacing between a pair of energy levels comprising the lower energy level and the upper energy level while said discrete section of paramagnetic material is in said first magnetic field beingsuch that transitions of electrons occur between the pair of levels comprising the lower energy level and the upper. energy level thereby creating an excess of electrons having high energy spin states and occupying the upper energy level, means adjacent the second resonant cavity for subjecting the discrete section ofparamagnetic material to a second magnetic field which is greater in. magnitude than thefirst. magnetic field as the section of paramagnetic material passes through the second resonant cavity, the direction of the second magnetic field being substantially parallel to that of the first magnetic field, and means for. conducting an electromagnetic wave energy signal of a second predi' termined frequency to and from the second resonant cavity, the second magnetic field being sufiiciently greater in magnitude-than the-first magnetic field whereby the separation between energy levelsof. a different pair of levels is greater while the paramagnetic material is in the second resonant cavity than the separation between the first named pair ofenergy levels whilethe paramagnetic; material is in the first resonant cavity, said dife ferentpair including one of. the energy levels of. the first named pair, the electrons of said paramagnetic material having transitions betweensaid. difierentpair of energy levels at said second frequency while in said second magnetic field of greater magnitude, the speed of movement of said paramagnetic section with respect to the spin-lattice relaxation times of electrons between said three energy levels being suchthat at least some of the electrons in the discrete paramagnetic section in the sec ond resonant cavity give up energy to the electromagnetic wave of said second. frequency before spin-lattice relaxation'is complete in the paramagnetic material of the last named section, the density of the electrons with high energy spin states in the discrete paramagnetic section and occupying the upper level of said different pair of levelsbeing-greater than that necessary to cause oscillation build-up in the second resonant cavity due to noiseand a signal of said second frequency, said speed of movement of the continuously moving material being preselected whereby said last named discrete section of paramagnetic material moves out of the second resonant cavity after a predetermined time interval to provide for. dampingof electromagnetic wave energy developed in said second cavity while the discrete section was (.liS posed therein, thebuild-up of energy in the second resonant cavity as discrete sections of paramagnetic material move through the second cavity providing for superregenerative pulsed amplification of the signal of said secondfrequency, the total length of said predetermined path and said predetermined. speed being chosen with respect to each other and the paramagnetic material whereby a discrete paramagnetic section after leaving the second resonant cavitytraverses a portion of at least a predetermined length of the path in returning to the first resonant cavity, the last-named discrete paramagnetic section remaining in said portion at least a sufiicient time to substantially produce a thermal distribution of the electron population between the energy levels therein.

ReferencesC-ited in the file of this patent UNITED STATES PATENTS 2,762,871 Dicke Sept. 11, 1956 2,802,944 Norton Aug. 13, 1957 2,836,722 Dicke et al May 27, 1958 2,879,439 Townes Mar. 24, 1959 2,909,654 Bloembergen Oct. 20, 1959 OTHER REFERENCES Basov et al.: Possible Methods, etc, I. Exper. Theoret. Phys. USSR 28, pages 249-250, February 1955.

Gordon: Hyperfine Structure, etc., Physical Review, vol. 99, No. 4, Aug.,l5, 1955, pages 12534263.

Wittke: Proceedings of the IRE, vol. 45, March 1957,

pages 291-316. 

